How time is measured from a town in England

If there's one thing that all the nations of the world agree on, it's time. This means that it is possible to know the time for any place on the globe. To do this we have to understand Greenwich Mean Time or GMT for short.

Greenwich Mean Time is universal time. It means all the clocks the world over are based on this one standard of time.

Greenwich (pronounced Grin-ij) is an actual place in England. It is just six miles downstream from London on the River Thames.

It was here in 1675 - more than 300 years ago - that King Charles II ordered the construction of an observatory to make accurate tables that would show the positions of the stars and the planets.

(From 1948 to 1957 the Royal Greenwich Observatory was moved from Greenwich to Herstmonceux Castle in Sussex to avoid interference from the bright lights of London).

In the days of King Charles II, ships' captains charted their courses by observing the stars in the night sky.

In 1884, almost 200 years later, an international conference of astronomers was held in Washington, D.C. to form a standard of time that could be used throughout the world. The astronomers agreed on what is called Greenwich Mean Time.

To understand Greenwich Mean Time we need to know about longitude. Meridians or lines of longitude are those lines that run north and south from Pole to Pole on any map of the earth's surface.

Although you see these lines clearly on the globe they don't really exist. They are imaginary, but they help us to calculate the time zones of the world.

The lines that go across the maps of the world from west to east like belts around the Earth's middle are lines of latitude. The words longitude and latitude come from Latin. Longitude means length and latitude means width. Although they are Latin words, it was the Greeks who introduced longitude and latitude about the 5th Century B.C.

The Earth is, in fact, the world's only real clock because it rotates on its axis at a constant speed everyday.

The local time for any place on the Earth depends upon its longitude. This is where Greenwich comes in. At that conference in Washington in 1884, it was agreed that Greenwich, England, would be the basis or starting point for calculating time.

In doing so the astronomers decided that the imaginary line passing through the Royal Greenwich Observatory would be called the prime meridian of the Earth. Therefore, Greenwich is located on 0 degrees longitude.

Knowing this we can now calculate all the time zones of the world. This is how it is done:

Because the globe is a circle or sphere it contains 360 degrees. There are 24 time zones (actually there are 23 full zones and two half zones making 24 in all) because there are 24 hours in every day.

When you divide 360 by 24 you get 15 degrees. This tells us the distance between lines of longitude is 15 degrees. Each time zone therefore represents 15 degrees.

To calculate the time for any place we add one hour for every 15 degrees of longitude for places east of Greenwich. For places west of Greenwich we subtract one hour for every 15 degrees of longitude. This means that Europe, for example, is ahead of Greenwich time. The United States on the other hand is behind GMT.

There is a place on the far side of the prime meridian (Greenwich) which is known as the International Date Line. It is in the Pacific Ocean on the 180 degree meridian.

If you are flying in an airplane or sailing in a ship and you cross this line , you are at once in a different day. Going westwards, say from the United States to Japan, you move ahead a day. Going eastwards from Japan to the US you have to repeat a day.

At the end of this month, if you live in the US, you will not gain an extra day, but an extra hour of sleep in the morning. This is because at 2 a.m. on Sunday Oct. 25 we move off Daylight Saving Time, which lasts from the last Sunday in April to the last Sunday in October.

If you are confused whether to take an hour off or add an hour on the clock in the morning, remember this well-known piece of advice: Spring Forward, Fall Back.